1. Field of Invention
This invention is related to microwave radiation and more particularly to a reflectarray antenna.
2. Description of the Prior Art
A type of antenna, known as a reflectarray antenna, is comprised of an array of radiators, such as dipoles, spirals, or waveguides. The radiators are respectively connected to ones of a plurality of phase shifters that are controlled by a computer. The reflectarray antenna is illuminated by a primary wave from a primary feed.
The primary wave passes through the phase shifters, is reflected back to the radiators, and re-radiated into space. The re-radiated wave has a phase front determined by the phase shifters. Additionally, as well known in the art, the re-radiated wave has only one polarization state. In other words, the far field of the re-radiated wave has only one fixed direction of polarization. Accordingly, the reflectarray antenna has some of the features of a reflector and others of a directly fed phased array antenna. Reflectarray antennas are described in a technical article "The Reflectarray Antenna" by Berry, Malech and Kennedy (November, 1963 IEEE Transactions on Antennas and Propagation) and in Section 11.8 of Radar Handbook by M. I. Skolnik, 1970 published by McGraw Hill, Inc.
When the polarization state of the primary wave is selectable, it may be desired to have the polarization state of the re-radiated wave determined by the polarization state of the primary wave, thereby achieving polarization flexibility. The polarization flexibility is achieved when the radiators each have a pair of feed ports where an absorbed wave is resolved into components having mutually orthogonal polarization states. The feed ports of a radiator are connected, respectively, to a pair of phase shifters whereby the antenna includes two phase shifters for each radiator. The orthogonality of the polarization states of the pairs of elements causes the array to be capable of forming and steering a wave having a polarization determined by the polarization of the primary wave. However, cost and power comsumption associated with two phase shifters for each radiator may be prohibitive.